The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD). A method for performing the analysis to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
Various types of position-sensitive X-ray detectors are known in the art of reflectometry. Solid-state arrays typically comprise multiple detector elements, which are read out by a CCD or other scanning mechanism. Each element accumulates photoelectric charge over a period of time before being read out and therefore cannot resolve the energy or number of incident X-ray photons. XRR using such arrays simply records the total integrated radiation flux that is incident on each element. Energy discrimination can be achieved only if an additional monochromator is used between the sample and the detector array, but this configuration results in signal throughput that is too low for practical applications.
Proportional counters are a type of gas-based, position-sensitive, X-ray detectors that do provide some energy resolution, typically about 20% (1200 eV for a 6 keV line). Such counters, however, are capable of processing only one photon at a time, leading to very slow analysis speed. Their energy resolution is inadequate for many applications.
Another common method of X-ray reflectometric measurement is described, for example, in an article by Chihab et al., entitled xe2x80x9cNew Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,xe2x80x9d in Journal of Applied Crystallography 22 (1989), p. 460, which is incorporated herein by reference. A narrow beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface in order to cut off the primary X-ray beam, so that only reflected X-rays reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector.
X-ray reflectometry has been combined with measurements of X-ray fluorescence (XRF) to provide additional information on the composition of thin film layers. For example, an article by Lengeler, entitled xe2x80x9cX-ray Reflection, a New Tool for Investigating Layered Structures and Interfaces,xe2x80x9d in Advances in X-ray Analysis 35 (1992), p. 127, which is incorporated herein by reference, describes a system for measurement of grazing-incidence X-ray reflection, in which X-ray fluorescence is also measured. A sample is irradiated by an X-ray source at grazing incidence. One X-ray detector captures X-rays reflected (likewise at grazing incidence) from the surface of the sample, while another detector above the sample captures X-ray fluorescence emitted by the sample due to excitation by the X-ray source. Analysis of the fluorescence emitted when the sample is excited at an angle below the critical angle for total external reflection of the incident X-rays, as described in this article, is known in the art as total reflection X-ray fluorescence (TXRF) analysis.
A related technique is described in an article by Leenaers et al., entitled xe2x80x9cApplications of Glancing Incidence X-ray Analysis,xe2x80x9d in X-ray Spectrometry 26 (1997), p. 115, which is incorporated herein by reference. The authors describe a method of glancing incidence X-ray analysis (GIXA), combining X-ray reflectivity and angle-dependent X-ray fluorescence measurements to obtain a structural and chemical analysis of a sample.
An alternative method for determining the thickness and composition of thin film layers is described in an article by Wiener et al., entitled xe2x80x9cCharacterization of Titanium Nitride Layers by Grazing-Emission X-ray Fluorescence Spectrometry,xe2x80x9d in Applied Surface Science 125 (1998), p. 129, which is incorporated herein by reference. This article describes a technique whereby a sample is irradiated by an X-ray source at normal or near-normal incidence, and fluorescent X-ray photons emitted by the sample are collected at a grazing angle, close to the surface. The spectrum of the collected photons is analyzed by a technique of wavelength dispersion, as is known in the art, and the distribution of photons by emission angle is determined, as well. The resultant data provide information about the thickness and composition of thin film layers on the sample.
Energy dispersion techniques can also be used to analyze the spectral distribution of reflected photons, as described, for example, in a paper by Windover et al., entitled xe2x80x9cThin Film Density Determination by Multiple Radiation Energy Dispersive X-ray Reflectivity,xe2x80x9d presented at the 47th Annual Denver X-ray Conference (August 1998), which is incorporated herein by reference.
X-ray detector arrays with a dedicated processing circuit for each detector have been developed for use in imaging systems based on synchrotron radiation. Such arrays are described by Arfelli et al., in articles entitled xe2x80x9cNew Developments in the Field of Silicon Detectors for Digital Radiography,xe2x80x9d in Nuclear Instruments and Methods in Physics Research A 377 (1996), p. 508, and xe2x80x9cDesign and Evaluation of AC-Coupled FOXFET-Biased, xe2x80x98Edge-onxe2x80x99 Silicon Strip Detectors for X-ray Imaging,xe2x80x9d in Nuclear Instruments and Methods in Physics Research A 385 (1997), p. 311, which are incorporated herein by reference. The detectors in the array are read by a VLSI CMOS circuit for multichannel counting, including a preamplifier, shaper, buffer, discriminator and counter for each channel. The detector array chip is connected to the VLSI inputs by wire bonding, although the authors state that a future redesign may make it possible to mount the front-end circuits directly on the detector chip itself.
It is an object of the present invention to provide improved methods and apparatus for position-sensitive X-ray detection.
It is a further object of some aspects of the present invention to provide improved methods and apparatus for energy-resolved X-ray analysis of a sample, and particularly for X-ray reflectometric analysis.
In preferred embodiments of the preferred embodiment, X-ray detection apparatus comprises an array of X-ray sensitive detectors, coupled to respective signal processing channels. Preferably, the detectors comprise photodiodes, as are known in the art, which are disposed in a linear or matrix (two-dimensional) configuration. The processing channels comprise integrated circuits, which are formed or mounted on a common substrate together with the respective detectors, so that each channel is coupled to its respective detector as an integral unit. Most preferably, all of these units are formed together on a single integrated circuit chip, but alternatively, the apparatus may be made up of a number of separate components, mounted on a hybrid, chip carrier or other printed circuit.
When an X-ray photon strikes one of the detectors, an electrical pulse is generated, having an amplitude indicative of the energy of the incident photon. The pulse is processed by the respective channel in order to determine the energy of the photon, as is known generally in the art of energy-dispersive X-ray signal processing. Each of the channels generates an output dependent on the rate of incidence of X-ray photons on the respective detector and the distribution of the energy of the incident photons. The sensitivity of the channels is automatically or manually controlled, typically based on adjustment of the time constant and gain of a pulse-shaping filter in each channel. Optionally, the sensitivity in each channel is controlled separately so as to increase the sensitivity of channels in which there is a relatively low rate of incident photons, while the sensitivity of channels having high incidence rates is reduced in order to allow high pulse throughput.
The array with parallel processing of the individual channel signals allows position-sensitive, energy-dependent X-ray photon counting to be performed with extremely high efficiency, energy resolution and dynamic range. These qualities cannot be achieved in detector arrays known in the art of X-ray reflectometry, in which multiple detectors share a common pulse processing channel, and only the total or average flux can be measured.
In some preferred embodiments of the present invention, the processing channels comprise energy level discriminators, which eliminate pulses due to photons of energy outside a predetermined range. The discriminators of all of the channels are preferably adjustable, either individually or all together, so that only photons within the predetermined range are counted.
In one of these preferred embodiments, the array is used to detect X-ray reflectivity from a sample, which is irradiated by an X-ray beam at a given, substantially monochromatic energy level. The discriminators are set to accept only pulses due to reflected photons, and to reject energy-shifted photons due to scattering and fluorescent processes. The use of the array thus enables accurate reflectance measurements to be made with high dynamic range and high throughput, while obviating the need for filtering or monochromatization of the beam reflected from the sample.
There is therefore provided, in accordance with a preferred embodiment of the present invention, apparatus for X-ray analysis of a sample, including:
an X-ray source, which irradiates the sample; and
an X-ray detector device, which receives X-rays from the sample responsive to the irradiation, the device including:
an array of radiation-sensitive detectors, which generate electrical signals responsive to radiation photons incident thereon; and
processing circuitry including a plurality of signal processing channels, each coupled to process the signals from a respective one of the detectors so as to generate an output dependent upon a rate of incidence of the photons on the respective detector and upon a distribution of the energy of the incident photons.
Preferably, the array of detectors includes an array of radiation-sensitive diodes, most preferably silicon diode detectors.
Further preferably, each of the plurality of signal processing channels includes an integrated circuit disposed on a common substrate with the respective detector. Most preferably, the common substrate includes a semiconductor chip including integrated circuits belonging to a multiplicity of the signal processing channels.
In a preferred embodiment, the signal processing channels process the signals in accordance with adjustable processing parameters, which are optionally individually adjusted responsive to different incidence rates of the photons at the respective detectors.
Preferably, the signal processing channels include discriminators, which reject signals corresponding to photons outside a predetermined energy range, wherein the processing circuitry includes a threshold control circuit, which adjusts the predetermined energy range of the discriminators.
Preferably, the signal processing channels include counters, which count the number of photons incident on the respective detectors responsive to the energy of the photons, and the processing circuitry includes a bus common to a multiplicity of the channels, which receives and outputs respective photon counts from the channels in turn.
In a preferred embodiment, the X-ray detector device receives X-rays reflected from the sample or, alternatively or additionally, fluorescent X-rays emitted by the sample. Preferably, the X-ray source includes a monochromator, such that the sample is irradiated with substantially monochromatic X-rays at a predetermined energy. Most preferably, the signal processing channels include discriminators, which are adjusted to reject signals corresponding to photons outside an energy range including the predetermined energy of the monochromatic X-rays.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for X-ray analysis of a sample, including:
irradiating the sample with X-rays;
receiving X-rays from the sample, responsive to the irradiation, at an array of detectors in respective, predetermined locations, which detectors generate electrical signals responsive to X-ray photons incident thereon; and
processing the signals from the array of detectors in respective processing channels, so as to generate an output indicative of a rate of arrival of the photons incident at the respective locations and dependent upon a distribution of the energy of the incident photons.
There is additionally provided, in accordance with a preferred embodiment of the present invention, radiation detection apparatus including:
an array of radiation-sensitive detectors, which generate electrical signals responsive to radiation photons incident thereon; and
processing circuitry including:
a plurality of signal processing channels, each channel coupled to process the signals from a respective one of the detectors and including a counter, which counts the number of photons incident on the respective detector; and
a bus common to a multiplicity of the channels, which receives and outputs respective photon counts from the channels in turn.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: